This application is the US national phase of international application PCT/GB00/03070 filed Aug. 9, 2000, which designated the US.
The present invention relates to Group III metal compounds, processes for their preparation and their use in deposition. The invention in particular relates to gallium compounds, processes for their preparation and their use, for example in the vapour deposition of a gallium layer or a III-V semiconductor layer.
The epitaxy of III-V semiconductors is used in the fabrication of very thin semiconductor layers. These layers are highly efficient because of the xe2x80x9cquantum wellxe2x80x9d effect. The layers are usually formed either by epitaxy from solution or by gas phase or molecular beam epitaxy.
Although epitaxy from solution is the most economical method, the size of the structure which can be obtained is severely limited and very thin structures cannot be fabricated. Molecular beam epitaxy is the most expensive method and is consequently used mainly for scientific research. As a result of the limitations of solution and molecular beam epitaxy, gas phase epitaxy is commonly adopted. Generally gas phase epitaxy is not very expensive and allows the fabrication of very thin structures. For example, the reaction of Ga(CH3)3 with AsH3 is used for the deposition of GaAs layers:
Ga(CH3)3(g)+AsH3(g)xe2x86x92GaAs(s)+3CH4(g)
However, Ga(CH3)3 is not an ideal source of gallium because the presence of a relatively strong Gaxe2x80x94C bond is liable to give rise to carbon impurities in the semiconductor layer which is formed. The efficiency of a semiconductor is significantly reduced by the presence of even small amounts of impurities. Therefore, there is a need to develop stable gallium compounds which are volatile and which decompose completely on pyrolysis at a surface to give gallium and volatile organic or other molecules.
The present invention provides stable Group III metal compounds which are volatile and which on decomposition give the Group III metal and other products which are stable and volatile. The compounds may be used, for example, in the vapour deposition of a Group III metal layer or a III-V semiconductor layer.
Accordingly the present invention provides a compound of formula (I) 
wherein
X is aluminium, gallium or indium,
each Y, which may be the same or different, is nitrogen or phosphorus;
R1 and R2, which may be the same or different, are hydrogen, halogen or alkyl; and
R3 to R7, which may be the same or different, are hydrogen or a saturated group,
or R3 and R4, or R5 and R6 together represent a saturated divalent link thus completing a ring.
The term xe2x80x9chalogenxe2x80x9d as used herein means fluorine, chlorine, bromine and iodine.
The term xe2x80x9calkylxe2x80x9d as used herein includes both straight and branch chain radicals. Typically it is C1 to C6 alkyl, preferably C1 to C4 alkyl, for example methyl, ethyl, i-propyl, n-propyl, t-butyl, s-butyl, or n-butyl. It may also be pentyl, hexyl and the various branch chain isomers thereof
The term xe2x80x9csaturated groupxe2x80x9d as used herein means a group in which the atoms are linked by single bonds. Typically it includes alkyl, silyl and trialkylsilyl.
The term xe2x80x9csaturated divalent linkxe2x80x9d as used herein means a chain in which the atoms are linked by single bonds, for example alkylene.
The term xe2x80x9cringxe2x80x9d as used herein typically includes five to eight membered rings, especially five and six membered rings.
X is preferably gallium.
Each Y is preferably nitrogen.
R1 and R2 are preferably hydrogen, chlorine or methyl.
R3 to R7 are preferably hydrogen, alkyl, silyl or trialkylsilyl. More preferably R3 to R7 are hydrogen, methyl, silyl or trimethylsilyl. Most preferably R3 to R7 are hydrogen or methyl.
A preferred compound of formula (I) according to the present invention is 1,1,3,3-tetramethylguanidine-gallane.
The compounds of formula (I) may be prepared by the reaction of a Group III metal hydride source with a base of formula (II) 
wherein Y and R3 to R7 are as defined above, or a salt thereof.
The Group III metal hydride source is typically LiXR1R2H2, R1R2HX or R1R2HXRNMe3 wherein X, R1 and R2 are as defined above. Group III metal hydride sources may be prepared by reaction of a suitable Group III metal compound with LiXH4 wherein X is as defined above. For example, gallium hydride adducts of a particular base may be prepared as follows: 
Salts of the bases of formula (II) suitable for use according to the present invention include both inorganic salts, for example the hydrochloric salt, the hydrobromic salt and the hydroiodic salt, and organic salts, for example the trifluoroethanoate salt. Preferably the salts of the bases of formula (II) are hydrochloride salts.
When a salt of a base of formula (II) is used, the process is generally carried out in the presence of a solvent. The process may be carried out in the absence of a solvent when a free base of formula (II) is used.
The process needs to be carried out with rigorous exclusion of air and moisture; it may be carried out in the gas phase or the liquid phase. Typically the process is carried out at low temperature, for example xe2x88x9278xc2x0 C., and under vacuum.
In one embodiment the compounds of formula (I) may be prepared commercially by producing a chemically unstable Group III metal hydride source, for example gallium hydride, in the locality of the preparation plant by methods known in the art. The hydride may then be treated with an amine, for example trimethylamine, to form a relatively storage-stable hydride-amine complex. A base of formula (II) can then be introduced into or mixed with the amine-hydride complex in the gas phase to displace the amine. Amines such as trimethylamine are very volatile leaving groups and allow the compounds of formula (I) to be condensed in high purity; the unused amine can be recovered and used for a subsequent batch.
The compounds of formula (I) can be used in deposition according to methods known in the art; The compounds may be used in vapour deposition to form a Group III metal layer or a III-V semiconductor layer. The compounds of formula (I) may be decomposed by, for example, thermolysis or photolysis. A III-V semiconductor layer may be prepared by decomposition of a compound of formula (I) with, for example, ammonia to form the Group III metal nitride layer, or with arsine, phosphine, stibine or other volatile compound of arsenic, phosphorus or antimony to form the Group III metal arsenide, phosphide or antimonide layer, respectively.